It is known that a spark ignition (SI) internal combustion (IC) engine is generally most efficient when the cylinder pressure and temperature at the end of a compression phase are closed to its maximum tolerable limit. In a conventional spark ignition engine, whether it is a rotary or a reciprocating one, this condition is achievable only when the throttle valve in the intake manifold is fully open to allow the maximum possible air or fuel-air mixture in the engine cylinder during intake phase and during following compression phase said intake air get compressed into a minimum chamber volume which is fixed by the design of the engine. During fully-open throttle condition the intake manifold pressure is near atmospheric pressure or about 1 bar. During the typical driving conditions which generally cover above 90% of the entire drive cycle, the intake manifold pressure remains about 0.5 bar or less, causing considerable drag on the driveshaft and this phenomenon is commonly known as ‘pumping loss’, that adversely affects the engine efficiency. Throttling further reduces chamber pressure and temperature at the end of compression phase and increase charge dilution. Hence reduces the combustion flame speed and the engine suffers from unstable combustion which leads to reduction in efficiency and increase in hazardous tailpipe emissions.
Conventionally, a mid-size car with a gasoline engine is only about 20% efficient when cruising on a level road whereas the rated peak efficiency of the car is about 33%. That is, during cruising, the Specific Fuel Consumption (SFC) of the engine is about 400 g/kWh, while under high load condition the same engine can reach a SFC of 255 g/kWh. See, P. Leduc, B. Dubar, A. Ranini and G. Monnier, “Downsizing of Gasoline Engine: an Efficient Way to Reduce CO2 Emissions”, Oil & Gas Science and Technology—Rev. IFP, Vol. 58 (2003), No. 1, pp. 117-118. As the engine operating condition goes below cruising mode such as the city driving conditions, the efficiency further reduces drastically. Considering this, if an engine is so downsized to operate with higher specific load during cruising or city driving condition, it could not accelerate or climb steep road well.
Ongoing research efforts, visible mostly in the reciprocating engine vicinity, indicate the future trends of improving thermodynamic efficiency of SI engine, which may also be extended to implement and to improve in case of Rotary engines as well under the same reference. Introduction of a fuel efficient Rotary engine, therefore, demands a quick review of the implementation of those efforts as being done in the field of reciprocating engines.
Throughout the past decades some interesting ideas like Variable Displacement Technology, Variable Compression Ratio Technology, Variable Valve Technology, Engine Downsizing and Pressure Boosting, Stratified Charging of Fuel, Controlled Auto Ignition, Load Dependant Octane Enhancement of Fuel have been introduced in order to attain better SI engine efficiency and various sets of combinations of these methods have also been experimented within a single engine.
In reciprocating piston engine the Variable Displacement volume of engine is generally achieved by cylinder deactivation method, wherein, during part load operation, few cylinders of a multi-cylinder engine are selectively deactivated so that not to contribute to the power and thus reducing the active displacement of the engine. Therefore, only the active cylinders consume fuel and are operated under higher specific load than that of the all cylinder operations, hence the engine attains higher fuel efficiency. The number of deactivated cylinders can be chosen in order to match the engine load, which is often referred to as “displacement on demand”. As pistons of both of the active and deactivated cylinders are generally connected to a common crankshaft, the deactivated pistons continue to reciprocate within the respective cylinders resulting in undesired friction. The valves of the deactivated cylinders need specialized controls, which produce further complications. Moreover, the deactivation and reactivation of cylinders take place in steps, and therefore further measures become necessary in order to make the stepped transitions smooth. Managing unbalanced cooling and vibration of variable-displacement engines are other designing challenges for this method. In most instances, cylinder deactivation is applied to relatively large displacement engines that are particularly inefficient at light load.
Modern electronic engine control systems are configured to electronically control various components such as throttle valves, spark timing, intake-exhaust valves etc. in order to smoothing of the transition steps of a variable displacement IC engine. An example of electronic throttle control method is to be found in U.S. Pat. No. 6,619,267 (Pao), describing the intake flow control scheme to manage the transition steps. A variable displacement system for both the reciprocating piston and rotary IC engines is disclosed in U.S. Pat. No. 6,640,543 (Seal) that includes a turbocharger to enhance the working efficiency.
A control system for a variable displacement internal combustion engine is to be found in JP2001115865 A (Arai Masahiro, Nagaishi Hatsuo) describing determination of effective flow cross sectional area in response to a throttle position. The effective flow cross sectional area is used to determine a volumetric airflow ratio. A control unit determines deactivation and reactivation of some of engine cylinders and varying strokes in a cycle. The control unit modifies the predetermined function in response to the number of cylinders being activated and the number of strokes in a current cycle. A rotary variable displacement volume engine is disclosed in WO 2006/042423 A1 (Pekau), wherein a rotary engine having a toroidal cylinder within which a set of pistons rotatable unidirectionally and coaxially about a driveshaft. A rotating disk valve with a partially cutoff portion sequentially intercept the toroidal cylinder to realize a compression phase when a piston is approaching the disc valve and an expansion phase when a piston is getting further from the disc valve. The cutoff portion of the rotating disk valve synchronizingly provides an opening so that at the end of compression the piston can pass the disk valve area. On the passing of the piston, said disc valve closes the toroidal cylindrical path in order to form an expansion chamber between the disc valve and the piston just passed the disc valve. A volume variable combustion chamber is fluidly connected to both compression and expansion chambers. Plurality of selectively operable intake and exhaust valves are arranged along the toroidal cylinder. Selective opening of particular intake valve or valves dictate the amount of intake air and similarly selective opening of exhaust valves dictates the expansion limit. In this engine design pumping loss could be avoided but it is very difficult to avoid a substantial loss of compressed air directly to the exhaust chamber during the opening of the disc valve. Moreover, hot gas flow from the separate combustion chamber to the expansion chamber could be led to high heat loss, over heating of duct and respective valves and seems to be very complex to control.
Like variable displacement engine technologies, the variable compression ratio (VCR) technologies also require various associated modifications such as engine downsizing, turbocharging or supercharging, variable valve technology, load dependant octane enhancement of fuel etc. to meet increasing stringent emission norms and fuel efficiency requirements. The basic VCR idea is to run an engine at higher compression ratio under part load operating conditions when a fraction of full intake capacity is consumed and at relatively lower compression ratio under heavy load conditions when the full intake capacity is consumed. Thereby the resultant cylinder pressure and temperature at the end of compression can be improved through a wide load conditions, hence, better fuel efficiency could be achieved. As VCR technology alone cannot avoid part load pumping losses, it requires assistance of Variable Valve Technology (VVT). The VVT provides the benefit of un-throttled intake to an SI engine, wherein the amount of intake gas at part load is controlled by either closing the intake valve early to stop excess intake or by late intake valve closing so that to discharge excess intake gas back to the intake manifold. The VCR technology itself, however, is quite complex to design and manufacture. See “Benefits and Challenges of Variable Compression Ratio (VCR)”, Martyn Roberts, SAE Technical Paper No. 2003-01-0398.
Over expansion cycle in a SI engine can add significant benefit to its thermal efficiency. The Atkinson cycle and Miller cycle efficiency is established on the said over expansion cycle principle, see “Effect of over-expansion cycle in a spark-ignition engine using late-closing of intake valve and its thermodynamic consideration of the mechanism”, S. Shiga, Y. Hirooka, Y. Miyashita, S. Yagi, H. T. C. Machacon, T. Karasawa and H. Nakamura., International Journal of Automotive Technology, Vol. 2, No. 1, pp. 1-7 (2001). The over-expansion cycle can produce substantial benefit in thermal efficiency over conventional engine cycle when being applied together with variable compression ratio and variable valve technology. But the introduction difficulties remain too high to introduce in a practicable engine.
The widely known conventional rotary IC engine, most familiar as the ‘Wankel engine’, has never been considered as an efficient engine because of some constraints inherent to its design, i.e. high surface to volume ratio of combustion chamber, high burning charge flow within the combustion chamber, uneven heating of the engine etc. Poor gas sealing capability and high lubricant contamination are other serious demerits of this engine. Mazda Motor Corporation of Japan continuing rigorous efforts for past few decades in order to improving the rotary engine efficiency and as a result considerable developed can be seen through various working components of the engine, such as increased intake-exhaust port area, introduction of sequential dynamic air intake system (S-DAIS), side exhaust ports for deducing exhaust gas overlapping into intake gas, reduced unburned Hydrocarbon emission, improved gas seals and combustion seals lubrication methods etc. See “Developed Technologies of the New Rotary Engine (Renesis)”, Masaki, Seiji, Ritsuharu, Suguru, Hiroshi-Mazda Motor Corp., SAE Technical Paper No. 2004-01-1790.
The purpose of the present invention is to propose a split cycle variable displacement engine which has continuous and wide range of displacement volume and compression ratio variation capacity; the engine is fairly simple to design and manufacture, easy to control and can maintain nearly full-load-like combustion environment (pressure, temperature, turbulence etc.) through the entire operating range.